1. Field of the Invention
This invention generally relates to improvements in the detection and differentiation of multifrequency signal. Specifically, a variety of filters are employed to selectively distinguish tone consisting of single frequency tone pairs. In the preferred embodiment, the filters, are optimized to detect all sixteen of the standard EIA R3464 DTMF tones with high selectivity and speed.
2. Description Of The Prior Art
The past several years have seen the exploitation of digital switch capabilities, particularly the ability to multiplex voice and data through the switch. A natural extension of this capability was the development of a mailbox capability for a user's telephone. This capability allowed much of the written correspondence associated with daily interaction to be replaced with voice communication stored and accessible from a person's phone.
A number of technological hurdles had to be overcome to provide this capability. Many of these dealt with the ability to effectively differentiate analog signals and filter them into their corresponding digital equivalents. This capability is referred to as digital signal processing. A detailed review of some of the modern approaches to digital signal processing is discussed in Stanley et al., "Digital Signal Processing", Reston Publishing Company, Inc. 1984. The discussion of nonrecursive (FIR) digital filters found in chapter 12 will be helpful in understanding the filtering techniques employed in the subject invention.
An example of an early dial pulse detector is disclosed in International Business Machines Technical Disclosure Bulletin, "Dial Pulse Detector", vol. 18, no. 4, pp. 991-2, Sept. 1975. The disclosed filter attentuates the low-frequency ringing of the dial pulse and improves the signal-to-noise and signal-to-audio ratios from a phone line to detect the dialed number. The processing logic of this system is less selective and much slower then the subject invention.
U.S. Pat. No. 4,369,338, to Souilliard, issued Jan. 18, 1983 discloses a tone detector for the sixteen standard DTMF pairs. The tone detector employs first and second tone detectors designed to provide logic signals in response to detection of predetermined frequencies. Received tones are processed by an active filter consisting of an RC network and amplifier. The filtered inputs are sent to the two tone detectors which are low power, phase-locked loop circuits whose frequency response is controlled by an RC network. The preferred embodiment employs an XR-L567 micropower tone decoder supplied by Exar Integrated Systems, Inc. which includes a quadrature phase detector in combination with the phase-locked loop. The paired filtering, provided to detect DTMF tone pairs, requires additional hardware not present in the subject invention and cannot accommodate the processing requirements for selectivity or speed that are provided by our method.
U.S. Pat. No. 4,620,294, to Cassopolis et al., issued Oct. 28, 1986 discloses a digital signal processor modem. The modem employs an analog-to-digital (A/D) converter to convert a quadrature differential phase keyed signal into a stream of digital pulse-code modulated (PCM) signals. U.S. Pat. No. 4,700,376, to Ohya et al., issued Oct. 13, 1987 discloses an automatic telephone answering apparatus with remote control capability for remote processing. As in the Cassopolis patent, a DTMF receiver is used for processing tone pairs. The two patents have primitive DTMF processors which lack selectivity and the processing speed or our invention.
An additional example of a DTMF receiver is disclosed in International Business Machines Technical Disclosure Bulletin, "Dual-Tone Multi-Frequency Receiver For Compressed Data", vol. 28, no. 9, pp. 4044-5. The article discloses a method which uses a time delay processing to improve selectivity. However, the selectivity and speed are insufficient for the purposes of the subject invention.